Theoretical model of the Au-Fe complex in silicon

We report results for the electronic states of the substitutional-gold--interstitial-iron complex in silicon. The self-consistent-field calculations were performed by using the multiple-scattering X{α} theory within the framework of the Watson-sphere-terminated molecular-cluster method. Guided by these investigations, and also taking into consideration recent theoretical studies for the isolated gold and iron impurities, we propose a microscopic model which provides a fairly consistent interpretation for the electron paramagnetic resonance (EPR) and diode capacitance measurements taken on the Au-Fe center. An effective spin S=(1/2) is assigned to the neutral charge state of the complex in agreement with EPR experiments. The model implies that the Au-Fe complex, as the isolated substitutional-gold center, has physical properties directly related to dangling-bond-like gap states. Furthermore the multiple-charge-state interpretation commonly invoked to explain the amphoteric behavior of the isolated gold impurity can explain the origin of the two deep levels observed for the complex. The suggested model is substantiated by the experimental fact that the Mott-Hubbard potential for the isolated gold and for the complex differ by only 0.11 eV.